a search for "Close my account" brought back 800 responses. I rest my case!
A keyword search for close my account (*without* the quotes) is searching for posts that have the word 'close' and/or the word account. To search for the exact phrase, search for "close my account" *with* the quotes. This will probably find exactly two posts, yours and mine.
From the display name menu (top right) if you select 'Your Posts' this is a list of all posts you have made on all forums sorted in descending order. As DC Ron said you can click on "Threads created", which will only show you posts where you started the thread (on the User Showcase this would be a song you posted).
Another option, if you want to see all of the posts you have made in the User Showcase forum specifically. Go to the Advanced Forum Search, and select only the User Showcase forum, and do a display name search for flwolf. This will show you ALL your posts in the User Showcase forum, not just the songs you have posted (i.e. the first post in the thread) - I can't think of a way to do that at the moment, however this single forum display name search should be pretty easy to filter through, as the title of replies start with "Re:"
A possible feature could be the ability to filter the Your Posts screen by a specific forum. It wouldn't be terribly difficult to add that feature.